1. Field of the Invention
The invention is related to an argon plasma apparatus and methods of using the argon plasma apparatus for cleaning, surface activation, etching and thin-film deposition.
2. Description of Related Art
Ionized gas plasmas have found wide application in materials processing. Plasmas that are used in materials processing are generally weakly ionized, meaning that a small fraction of the molecules in the gas are charged. In addition to the ions, these plasmas contain reactive species that can clean, activate, etch and deposit thin films onto surfaces. The temperature in these weakly ionized gases is usually below 250° C., so that most thermally sensitive substrates are not damaged. The physics and chemistry of weakly ionized plasmas are described in several textbooks. See for example, Lieberman and Lichtenberg, “Principles of Plasma Discharges and Materials Processing”, (John Wiley & Sons, Inc., New York, 1994), and Raizer, Y. P., “Gas Discharge Physics”, (Springer-Verlag, Berlin (1991).
According to the literature, weakly ionized plasmas are generated in vacuum at pressures between 0.001 and 1.0 Torr (see Lieberman and Lichtenberg (1994)). Electrical power is applied across two electrodes to break the gas down and ionize it. The electricity may be provided as a direct current (DC), alternating current (AC), radio frequency (RF), or microwave (MW) source. The electrode may be constructed to provide either capacitive or inductive coupling to strike and maintain the plasma. In the former case, two conducting electrodes are placed inside the vacuum chamber filled with a small amount of gas. One of the electrodes is powered, or biased, by the RF generator, while the other one is grounded. In the latter case, the RF power is supplied through an antenna that is wrapped in a coil around the insulating walls of the chamber. The oscillating electric field from the coil penetrates into the gas inducing ionization.
Over the past fifteen years, atmospheric pressure plasmas have been developed as an alternative to vacuum plasmas. These plasmas can treat an object of any size and shape, since they do not have to be loaded into a vacuum chamber. This can significantly reduce the cost of the process. A number of different atmospheric pressure plasma devices have been developed (Schutze, et. al., “The atmospheric-pressure plasma jet: A review and comparison to other plasma sources”, IEEE Trans. Plasma Sci. 26, 1685-1694 (1998). These plasmas are governed by how the ionization process is controlled. At atmospheric pressure, the gas density is so high that the ionization reaction can easily run away and generate a high temperature arc, which is not useful for materials processing.
There are three common types of atmospheric pressure plasmas used to treat materials. These include a dielectric barrier discharge (DBD), a torch, and a radio-frequency, noble gas discharge. The DBD has long been employed to treat rolls of plastic film whereby the material is continuously passed between the electrodes. In some instances, the DBD may be deployed as a downstream device, so that 3-D objects can be treated with the reactive gasses that flow out from between the electrodes. The torch and the RF noble-gas discharge are strictly downstream plasmas. Here, the ions and electrons are confined to the gap between the electrodes, and the substrate is exposed to a beam of neutral reactive species that exits from the source. A robot is used to scan the plasma beam over the substrate surface which is positioned <1 cm below the plasma. In contrast to vacuum plasmas, only the area on the sample surface requiring treatment is exposed to the reactive gas species.
The dielectric barrier discharge is usually operated with air (See Goldman and Sigmond, “Corona and Insulation,” IEE Transactions on Electrical Insulation, EI-17, no. 2, 90-105 (1982) and Eliasson and Kogelschatz, “Nonequilibrium Volume Plasma Chemical Processing”, IEEE Transactions on Plasma Science, 19, 1063-1077, (1991)). A 10 KV power supply operating at approximately 20 KHz supplies the voltage necessary to breakdown the gas. A dielectric barrier covers one of the electrodes, and prevents a high current arc from forming. During operation charges build up on the surface of the insulator and discharge as tiny “micro-arcs” within each AC cycle. The micro-discharges occur randomly in space and time, lasting for periods of 10 to 100 ns. Inside the micro-discharge, the electron density is high, whereas outside it is extremely low. Consequently, it is not possible to measure an average electron density and electron temperature for the entire gas volume between the electrodes. Note that substrates placed downstream of the DBD will not be treated uniformly with the plasma on the micro scale. In addition, the discharge can interact electrically with the substrate, making it difficult to treat components containing metals.
The torch is generated by forming an arc between closely spaced powered and grounded electrodes. This construct is described by Fauchais and Vardelle, in their article: “Thermal Plasmas”, IEEE Transactions on Plasma Science, 25, 1258-1280 (1997). Air is passed between the electrodes, and ionized by applying 10 kV AC power. The arc is a thermal plasma in which the neutral temperature is many thousands of degrees. Nevertheless, it is possible to blow gas through the arc at a sufficient velocity such that the overall gas temperature is low enough to treat thermally sensitive materials, including polymers. The Plasmaflume™ by PlasmaTreat is an example of this type of construct. It utilizes a rotating, cone-shaped electrode that rapidly spins the arc through the flowing gas volume, maintaining the average neutral temperature below 700 K. Plasma streamers shoot out the end of the housing and treat objects placed a short distance below.
Atmospheric pressure, noble gas plasmas, driven with radio-frequency power at 13.56 or 27.12 MHz, operate in a different way than the DBD and torch plasmas. These plasmas, sometimes referred to as atmospheric pressure plasma jets (APPJ), are weakly ionized, capacitive discharges (see Jeong et al., “Etching Materials with an Atmospheric-Pressure Plasma Jet,” Plasma Sources Science Technol., 7, 282-285 (1998); Babayan et al., “Deposition of Silicon Dioxide Films with an Atmospheric-Pressure Plasma Jet,” Plasma Sources Science Technol., 7, 286-288, (1998); Moravej, et al., “Physics of High-Pressure Helium and Argon Radio-Frequency Plasmas”, J. Appl. Phys., vol. 96, p. 7011 (2004); Babayan and Hicks, U.S. Pat. No. 7,329,608, Feb. 12, 2008, and Babayan and Hicks, U.S. Pat. No. 8,328,982, Dec. 11, 2012). The ions and electrons uniformly fill the gas volume between the metal electrodes, with a collisional sheath forming at the boundaries to repel the electrons and maintain the plasma. The average electron density and temperature in the RF, noble gas plasma has been determined to be 1011 to 1012 cm−3 and 1 to 2 eV, respectively. Depending on the RF power level, the neutral gas temperature ranges from 323 to 573 K. Molecular gases are fed with the helium or argon at concentrations from 2.0 to 5.0 volume %.
Non-thermal argon plasmas can be generated at atmospheric pressure as well (see Moravej, et al., “Physics of High-Pressure Helium and Argon Radio-Frequency Plasmas”, J. Appl. Phys., vol. 96, p. 7011 (2004). However, they are much more difficult to stabilize than the helium discharge for several reasons. The cross section is large, comparable to that of nitrogen and oxygen, yielding a fast rate of ionization. Secondly, the mass of argon is twenty times greater than helium, which reduces the electron mobility in the gas. In order to maintain a reasonable current through the argon discharge, the electron density must be pushed to a value greater than 1012 cm−3, where the plasma transitions from alpha- to gamma-mode ionization and becomes unstable (see Yang, et al., “Comparison of an Atmospheric Pressure, Radio-Frequency Discharge Operating in the Alpha and Gamma Modes”, Plasma Sources Sci. Technol., vol. 14, p. 314 (2005); and Shi and Kong, “Mechanisms of the Alpha and Gamma Modes in Radio-Frequency Atmospheric Glow Discharges”, J. Appl. Phys., vol. 97 (2005)).
In view of the foregoing, there is a need for an atmospheric pressure, plasma that operates in a stable mode with a uniform distribution of the ionized gas over the discharge volume. Further there is a need for such devices and methods for plasma that remains stable during operation over a wide range of conditions and with different gases. Such an plasma apparatus should provide higher fluxes of reactive species to increase the speed with which materials are processed, and may be rapidly scanned over surfaces to treat parts that are large and/or three dimensional, and are not easily processed in a vacuum chamber. These and other needs are met by embodiments of the present invention as described hereafter.